1. Field of the Invention
The present invention relates to a light scanning device using a semiconductor laser as a light source, and more particularly to an optical system of a light scanning device used in a laser printer, etc.
2. Background of the Prior Art
Heretofore, there is known a light scanning device using a semiconductor laser as exemplified in FIG. 4.
As is shown in FIG. 4, this light scanning device comprises a semiconductor laser 1 for emitting a light beam, a collimate lens 2 for collimating the light beam, a polygon mirror 3 rotatable about an axis l, an f.theta. lens 4 for imaging the light beam reflected by the polygon mirror 3 on a photosensitive drum 5 as a subject to be scanned.
The light beam emitted by the semiconductor laser 1 enters, at various angles, into the f.theta. lens 4 according to the rotation of the polygon mirror 3. As a result, a spot S imaged on the photosensitive drum 5 is scanned in a principal scanning direction m to form a series of dots on the photosensitive drum 5 according to the output of the semiconductor laser 1.
The photosensitive drum 5 is rotated in a secondary scanning direction n, and an output image is formed, as a collection of dots, by the aforementioned action.
Now, two characteristic points of a light beam emitted by the semiconductor laser will be described.
First, the shape of a light beam is oval (elliptical), as shown in FIG. 5, having a ratio of 1:1.5.about.3 between a short axis parallel to a direction x of a p-n junction and a long axis perpendicular thereto. Second, the polarizing state thereof is a linearly polarized light which vibrates within an x-z plane including the direction x in which an electric field vector is in parallel with the p-n junction and a progressing direction z of the light beam.
The shape of the spot S imaged on the photosensitive drum 5 is preferably oval (elliptical) having a short axis parallel to the principal scanning direction m in order to form a clear output image on the photosensitive drum 5. Furthermore, in order to form such a shape as mentioned on the photosensitive drum 5, it is necessary that an oval (ellipse) shaped light beam having a short axis parallel to the secondary scanning direction enters into the f.theta. lens 4. As the diameter of the spot where the beam is condensed is proportional to the inverse of the diameter of the incident light beam, the long axis at the point of incidence becomes the short axis on the image surface.
In this device, therefore, the semiconductor laser 1 is disposed as such that the short axis of the oval (ellipse) shaped light beam emitted by the semiconductor laser 1 is in parallel with the rotary axis l of the polygon mirror 3, while the short axis of the oval (elliptical) shaped spot S is parallel with the principal scanning direction m.
With the above-mentioned arrangement, however, since a vibrating plane A (FIG. 6) of the electric field vector of the light beam entering into the f.theta. lens 4 becomes perpendicular to an incident plane B including an incident optical path of the light beam and an incident normal of an end face C at an incident side of the f.theta. lens 4, the light beam becomes an s-polarized light when entering outside the optical axis of the f.theta. lens 4. The light beam entering as an s-polarized light is comparatively large in change of the transmittance due to the change of this incident angle compared with a p-polarized light or a circularly polarized light.
Specifically, when a monolayer four kinds of anti-reflection coating is applied to the end face C of the f.theta. lens 4, the transmittance changes as shown by broken lines in FIG. 7, and when a monolayer 780 of the anti-reflection coating is applied, it changes as shown by broken lines in FIG. 8.
Therefore, in the above-mentioned device, the light beam reaching the photosensitive drum 5 is greatly changed in intensity depending on whether the light beam enters into the f.theta. lens 4 on the optical axis thereof or outside the optical axis.
In this way, if the light beam passing through the f.theta. lens 4 is not uniform in light quantity, an output image formed on the photosensitive drum 5 also becomes nonuniform. Therefore, in a laser printer, for example, the image that is printed on paper is non-uniform.